1. Field of the Invention
This invention is in the field of tools used to cut through casing strings installed in a well, such as an oil and gas well, to allow the removal of the casing strings from the well.
2. Background Information
When an oil or gas well is drilled, it is common to install large pipe, or casing, in the hole as it is drilled. Several different diameters may be installed, in fact, in the same hole. Some of the casing strings can be large diameter, up to 20 or 30 inches. Often the casing is cemented together. Where multiple strings are to be cut and removed from the hole, the operator may elect to use a smaller bodied casing cutter tool, along with multiple sets of blades of different lengths, or one set of long blades may be chosen. As a typical example, an 113/4 inch outside diameter tool might be used in cutting multiple strings of casing having diameters of 133/8 inches, 20 inches, and 30 inches. The blades pivot near the top of the blade, to swing outwardly from the cutter tool. Where one set of long blades is used to make the full cut, the blades must be long enough to reach to the outer casing limits.
Other large diameter pipes are also used in the oil field, such as conductor pipes, or offshore platform legs. These can reach 60 inches in diameter. In cutting these larger diameter pipes, the operator may, again, elect to use a smaller bodied casing cutter tool, along with long blades.
Torque is transferred to the blade only at the top of the blade, where the blade is within the cutter body. This torque imposes an excessive bending stress or shear stress on the blade, because it is unsupported for most of its length, especially when cutting the larger diameter casing or pipe. Often in such a situation, the blade bends or breaks. Being much smaller than the casing diameter, the cutter body tends to wobble from side to side as the cut is being made. This tends to put enormous stresses on the blades, contributing to their failure. Further, when longer blades are used, the blade must be essentially straight, because the blade must fit within the outside diameter of the cutter tool to facilitate running the cutter tool into the hole. This means the blade will have a "sweeping" motion, causing longer blades to contact the casing at shallower angles, requiring the removal of more metal, and taking longer to make the cut.
If the blade bends excessively during the cutting operation, it may not retract fully into the cutter body. If the cutter must be removed from the well prior to completing the cut, the bent blades will not allow the cutter to be withdrawn up into the smaller casing, where present, and the cutter becomes stuck in the hole.
In an effort to strengthen the blades, different materials offering higher strength have been used, but this creates another problem. That is, the blades become more susceptible to weld cracks. The cracks contribute to breaking of the blades, leaving junk in the well, and, of course, not making the cut.
To reduce the tendency of the blades to bend or break, the operator often makes several runs using progressively longer arms. Some have also addressed this problem by simply building several tools with larger outside diameters. This adds to inventory, and still does not provide the ideal tool for torque resistance at the top of the blade. In most cases, the operator will at least add several stabilizers to his bottom hole assembly, to reduce the wobbling of the casing cutter tool.
It would be desirable to have a tool which supports the blade all the way to a position near the inside diameter of the pipe being cut, with the ability to cut several different diameter pipes, without having to stock multiple casing cutters in various tool body diameters.